The purpose of this project is a detailed investigation at the molecular level of phosphoglucose isomerase and phosphomannose isomerase, both of which have fructose-6-phosphate as the product of their forward reaction, with the aim to obtain specific and quantitative information with respect to similarities and differences in their mechanism of action. Both enzymes form the identical product in one direction of their catalyzed reaction. Since the products formed in the other direction distinguish themselves only by a different steric configuration of one hydrogen at one carbon, which might be considered the least variation necessary for two distinguishable reactions, the two reactions studied present an ideal object for the study of enzyme specificity. The aim, therefore, is to elucidate the structural features of the two enzyme proteins which enable them to distinguish between these very slight differences in the substrate(s) to be attacked. The approach to the problem includes isolation of both isomerases, their chemical and physical characterization, a thorough kinetic analysis of their respective catalyzed reactions, elucidation of their active site structures, investigation of the conditions required for their active conformations and evaluation of factors controlling their quarternary structures, use of 'reporter-group' labels to monitor environmental changes in or near the active center, synthesis of modified substrate analogues with chemically reactive groups to form covalently linked ES-compounds, and comparative study of suitable non- enzymatic model reactions. Also, a joint project on parallel investigations of the three-dimensional structure of phosphoglucose isomerase by chemical means and by X-ray diffraction analysis has been initiated with the University of Bristol (England) X-ray crystallography group.